1. Field of the Invention
The present invention relates to a light emitting display device in which light emitting elements constituting pixels are actively driven for example by TFTs (Thin Film Transistors) and in particular to a light emitting display device which can effectively restrain intensity non-uniformity among respective pixels which occurs due to variations in characteristics of light emission drive transistors that give drive current to the respective light emitting elements.
2. Description of the Related Art
Demand for a display panel which has a high definition image display function and which can realize a thin shape and low power consumption has increased due to popularity of cellular telephones, personal digital assistants (PDAS), and the like, and conventionally a liquid crystal display panel has been adopted in many products as the one which meets the needs thereof. Meanwhile, these days a self light emitting display panel utilizing an organic EL element whose characteristic as a self light emitting type display element is best used has been manufactured, and this have attracted attention as a next generation display panel in place of the conventional liquid crystal display panel. A background thereof is that by employing, in a light emission functional layer of the element, an organic compound which enables an excellent light emission characteristic to be expected, a high efficiency and a long life which can be equal to practical use have been advanced.
The organic EL element is constructed by laminating a transparent electrode for example by ITO, a light emission functional layer, and a metallic electrode one by one basically on a transparent substrate such as glass or the like. The light emission functional layer may be a single layer of an organic light emitting layer, or a double layer structure composed of an organic positive hole transport layer and an organic light emitting layer, or a triple layer structure composed of an organic positive hole transport layer, an organic light emitting layer, and an organic electron transport layer, or a multilayer structure in which an injection layer of electron or positive hole is inserted into an appropriate portion among these layers.
The organic EL element can be electrically replaced by a structure composed of a light emitting component having a diode characteristic and a parasitic capacitance component which is connected in parallel to this light emitting component, and thus the organic EL element can be said to be a capacitive light emitting element. When a light emission drive voltage is applied to this organic EL element, at first, electrical charges corresponding to the electric capacity of this element flow into the electrode as a displacement current and are accumulated. It can be considered that when the drive voltage then exceeds a determined voltage (light emission threshold voltage=Vth) peculiar to this element, current begins to flow from one electrode (anode side of the diode component) to an organic layer constituting the light emitting layer so that the element emits light at an intensity proportional to this current.
As a display panel employing such organic EL elements, a passive matrix type display panel in which EL elements are simply arranged in a matrix pattern and an active matrix type display panel in which active elements for example constituted by TFTs are added to respective EL elements arranged in a matrix pattern have been proposed. The latter active matrix type display panel can realize low power consumption compared to the former passive matrix type display panel and has a characteristic that crosstalk among pixels is small, whereby it is particularly suitable for a high definition display constituting a large screen.
FIG. 1 shows an example of a circuit structure corresponding to one pixel 10 in an active matrix type display panel already proposed. The circuit structure of the pixel 10 shown in this FIG. 1 shows an example in which a lighting drive method called the SES (Simultaneous Erasing Scan) method which realizes time division gradation expression is adopted.
In the structure of this pixel 10, a data signal Vdata corresponding to a video signal supplied from a data driver 11 is supplied to source S of a scan selection transistor, that is, a data write transistor Tr2, via a data line arranged in a display panel. A data write signal Write is supplied from a scan driver 12 to the gate G of the data write transistor Tr2 via a scan selection line.
The drain D of the data write transistor Tr2 is connected to gate G of a light emission drive transistor Tr1 and to one terminal of a light emission maintaining capacitor C1. The source S of the light emission drive transistor Tr1 is connected to the other terminal of the capacitor C1 and to an anode side drive power source Va. Further, the drain D of the light emission drive transistor Tr1 is connected to anode terminal of an organic EL element E1 provided as a light emitting element, and cathode terminal of this organic EL element E1 is connected to a cathode side drive power source Vc.
The pixel structure shown in FIG. 1 is provided with an erase transistor Tr3, and an erase signal Erase is supplied from an erase driver 13 to the gate of this erase transistor Tr3 via an erase signal line. The source S and drain D of the erase transistor Tr3 are connected to end portions of the light emission maintaining capacitor C1, respectively.
In the pixel 10 shown in FIG. 1, only the light emission drive transistor Tr1 is constituted by p-channel type TFT, and other transistors are constituted by n-channel type TFTs. A large number of the pixels 10 of the above-described structure are arranged in a matrix pattern in a row direction and a column direction to construct the display panel.
In the structure of the pixel 10 shown in FIG. 1, an ON voltage Write as a scan signal is supplied from the scan driver 12 to the gate of the data write transistor Tr2 during an address period. Thus, current corresponding to the data signal Vdata supplied from the data driver 11 flows in the light emission maintaining capacitor C1 via the source and drain of the data write transistor Tr2 so that the capacitor C1 is charged. Its charge voltage is supplied to the gate of the light emission drive transistor Tr1, and the transistor Tr1 allows drain current Id corresponding to a gate-to-source voltage (Vgs) which is based on the gate voltage thereof and on the drive power source Va supplied to the source to flow in the EL element E1, whereby the EL element E1 emits light.
When the gate of the data write transistor Tr2 becomes an OFF voltage after the address period elapses, the transistor Tr2 becomes in a so-called cut-off state. However, the gate voltage of the light emission drive transistor Tr1 is maintained by electrical charges accumulated in the capacitor C1, and thus drive current to the EL element E1 is retained. Accordingly, a lighting state of the EL element E1 which corresponds to the data signal Vdata can be continued until a period to a next address operation (for example, a next one frame period or a next one subframe period).
Meanwhile, in the middle of the lighting period of the EL element E1 (for example, in the middle of one frame period or one subframe period), the erase signal Erase which turns the erase transistor Tr3 on is supplied from the erase driver 13. In the case where this erase transistor Tr3 is turned on, electrical charges charged in the capacitor C1 are erased (charged) instantly, and as a result, the, light emission drive transistor Tr1 becomes in the cut-off state, whereby the EL element E1 is instantly extinguished.
In other words, by controlling output timing of the gate-on voltage supplied from the erase driver 13, the lighting period of the EL element E1 for example during one frame or one subframe period is controlled, and thus multi-gradation expression can be realized. The structure of a pixel provided with the erase transistor Tr3 in addition to the data write transistor Tr2 and the light emission drive transistor Tr1 as described above is disclosed in the following Japanese Patent Application Laid-open No. 2001-343933.
In many of these types of light emitting elements represented by the organic EL element have a current dependency that the light emission intensity is determined in response to the drive current. Meanwhile, regarding the light emission drive transistor employed in the above-described pixel structure, variations occurs in the characteristic of drain current Id with respect to the gate-to-source voltage Vgs, particularly in the characteristic of the gate-to-source voltage Vgs, that is, of the threshold, at which the drain current Id begins to flow. Thus, even though the same level of data signal Vdata is supplied, variations in light emission intensities occur among pixels.
Such variations in light emission intensities among pixels allow, particularly when an animation image and the like is reproduced, a vague stripe pattern or a phenomenon resembling flicker to be generated, thereby causing a problem that display quality is considerably degraded. Thus, in order to solve the problem, it is necessary to make the characteristic of TFTs formed in a display panel uniform, and regarding this, conventionally, various discussions and developments have been carried out. However, regarding this, there exist a number of technical problems including a problem of selection of a semiconductor material or other materials, a problem of a manufacturing process, a problem of a manufacturing environment, and the like, and it is difficult to pursue a fundamental solution.